1. Field of the Invention
The present invention relates to capping layers for multilayer reflective coatings used in extreme ultraviolet or soft x-ray lithography applications.
2. Description of Related Art
Extreme ultraviolet (EUV) and soft x-ray projection lithography make use of optical elements with highly reflective multilayer coatings. These multilayer coatings typically consist of alternating layers of molybdenum (Mo) and silicon (Si) or molybdenum and beryllium (Be). High EUV reflectivity is essential for lithography applications. A critical limitation to achieving the maximum theoretical peak reflectivity is the oxidation and corrosion of the top layers, which both increases the absorption and degrades the phase coherence of the reflection from these layers.
Multilayers built to reflect photons in the soft x-ray to ultraviolet regime decrease in reflectance upon exposure to oxygen due to an oxidation of the multilayer surface and/or an oxidation of the bulk of the multilayer caused by diffusion and bonding of oxygen. The presence of the oxygen decreases reflectance since it is heavily absorbing in this energy regime. The problem to be solved is stopping the oxidation of the multilayer surface and bulk.
Although there have been numerous investigations of carbon-based, boron carbide-based, and silicon-based multilayer coatings for EUV mirrors, there has been little work on environmental effects (e.g., oxidation and corrosion) of these structures. Underwood et al. (Applied Optics 32:6985 (1993)) investigated the aging effects of Moxe2x80x94Si multilayers by monitoring the decrease in reflectivity with time. Their experimental results showed a degradation of the Moxe2x80x94Si multilayer reflectance caused by the oxidation of the topmost layer of molybdenum. Underwood et al. identified the oxidation of the molybdenum layer as a potential problem in soft x-ray projection lithography. The proposed solutions were to make silicon the topmost layer, to store the optical elements in an inert atmosphere or vacuum, or to remove the oxidized surface by sputtering or chemical etching. Underwood et al. did not investigate the use of passivating layers.
Mo/Si multilayers with Mo as the top layer have the highest theoretically possible reflectivity; however, Mo is not stable in air and therefore Mo/Si multilayers for EUV optics are usually capped with a Si top layer with a loss in reflectivity of 1.3%. After exposure to air, this layer partly oxidizes and forms SiO2 that absorbs EUV light and reduces the reflectance of the multilayer by about another 1-2%. This reflectance of Si capped multilayers will remain unchanged for years if the multilayers are kept at room temperatures. See C. Montcalm, S. Bajt, P. B. Mirkarimi, E. Spiller, F. J. Weber, and J. A. Folta, in xe2x80x9cEmerging Lithographic Technologies IIxe2x80x9d, ed. Y. Vladimnirsky, SPIE Vol 3331,42-51 (1998). However, in a working EUV lithography tool the coatings are exposed to EUV illumination in the presence of low pressure background gases including water, oxygen, and hydrocarbons. L. Klebanoff et al., M. Wedowski et al. references have shown that the reflectance of Si capped Mo/Si multilayers decreased as a function of EUV illumination dose and the amount of water vapor and other background gases in the system.
In the soft x-ray and extreme ultraviolet regime of the spectrum, all materials adsorb. In order to reflect any appreciable amount of light at these wavelengths, multilayer structures must be used and matched to the desired reflected wavelength The physical principles that makes the multilayer structures reflect are the use of low index to high index interfaces and the spacing of the interfaces such that the small amount reflected from an individual interface constructively adds with the reflections from the other interfaces. This constructive interference allows for theoretical reflections higher than 10% from a 40 bilayer structure (13.4 nm desired reflected wavelength). The key to such high reflectivities is two fold: the physical structure of the multilayer stack and the materials used in its construction.
Ideally, the materials used would be non-adsorbing and environmentally stable when exposed to other elements in a variety of conditions. Unfortunately, all materials adsorb to varying degrees at these wavelengths. The key is building a multilayer structure that has large index of refraction differences between layers and has the interfaces between those layers to be very abrupt In order to preserve the high reflectivity achieved by such structures, the multilayer stack must be environmentally stable when subjected to a high flux of radiation in a hostile environment Traditionally silicon or a similar material has been used as the top most layer (or capping layer) of multilayer structures due to the formation of a passivating oxide when it is exposed to atmosphere at room temperature.
Silicon and other similar materials may provide sufficient protection for the multilayer structure at high fluxes of radiation in a hostile environment due to the formation of secondary electrons created by the incoming radiation. These secondary electrons effectively mimic a high temperature environment and can cause water on the surface to split into oxygen and hydrogen. The oxygen can then penetrate through the original passivating layer and cause the multilayer stack to oxidize. The oxidation of the multilayer stack destroys its high reflectivity by changing the indexes of refraction of the layers and by causing greater adsorption of the incoming radiation. Thus, a high reflectance multilayer structure in the soft x-ray/extreme ultraviolet regime can only be used in the absence of water, oxygen, and other hostile compounds or the multilayer structure must be protected from exposure to the hostile compounds by the use of an effective capping layer.
U.S. Pat. No. 5,958,605, titled xe2x80x9cPassivating Overcoat Bilayer For Multilayer Reflective Coatings For Extreme Ultraviolet Lithographyxe2x80x9d, discloses a passivating overcoat bilayer that is used for multilayer reflective coatings for extreme ultraviolet (EUV) or soft x-ray applications to prevent oxidation and corrosion of the multilayer coating thereby improving the EUV optical performance.
U.S. Pat. No. 5,080,862, titled xe2x80x9cIridium Silicon Alloyxe2x80x9d teaches an alloy having a very high resistance to oxidation. The alloy contains between 30 and 75 atom percent of silicon in an iridium base. The alloy may be used in the form of a surface coating to protect structural elements of other materials from oxidation. The alloy may also be used as an ingredient of a composite.
It is an object of the present invention to provide capping layer designs that use iridium and iridium compounds for the prevention of bulk oxidation and/or surface oxidation of multilayer coatings.
It is another object of the invention to provide control over the oxidation thickness of the capping layer of molybdenum/silicon thin films by replacing the top of the silicon capping layer with iridium silicide.
Another object of the invention is to provide control over the oxidation thickness of the capping layer of molybdenum/silicon thin films by terminating the thin film with molybdenum and then depositing or forming iridium molybdenide (IrMox) as a capping layer.
Still another object of the invention is to prevent oxygen diffusion into the thin film structure by use of iridium compound(s) as a capping layer.
These and other objects will be apparent to those skilled in the art based on the disclosure herein.
Iridium and/or iridium compounds are used as the capping layer on multilayer structures designed for use in the soft x-ray to extreme ultraviolet regime of the spectrum The use of iridium as part of the capping layer allows for greater versatility in the processes used to dean or pattern the surface of the capping layer. The use of an indium compound as a capping layer could stop the iridium from oxidizing and provide a non-permeable, passive barrier assuming the lattice spacing is small enough.
An optimized capping layer is formed of indium or iridium compound used with an optically less absorbing material (in the deep ultra-violet/soft x-ray regime). This optimization counters the absorption of the iridium and allows for the use of indium or iridium compounds as a capping layer without decasing the reflectivity by more than a few percent This application teaches solutions to problems that might arise with the use of iridium and/or iridium compounds as a capping layer. Some of the problems solved are: (i) the diffusion of the iridium into the multilayer stack, (ii) long term oxidation of the iridium that would allow water to permeate the capping layer and begin oxidizing the multilayer stack, and (iii) a large decrease in reflectivity of the multilayer stack caused by radiation being adsorbed by the capping layer.